My AD8310 log amp circuit only differs from the basic connection shown in the data sheet by having 200pF connected to OFLT pin, 680nF input coupling, and I have connected the dc Vout pin4 directly to Vin+ of a 22bit ADC ( not ADI ).
My circuit is built on double sided pcb, ground plane both sides, smt components and recommended decoupling; however no metal box is used for shielding/screening.
I use input matching for measuring power in a 50Ohm circuit with the recommended parallel 52R3 across the input; good for LF, HF frequencies. But for higher frequencies, VHF and up, I am not using any high pass input matching network .
I have calibrated my circuit using a 2-point calibration at 4MHz using 0dBm and -20dBm inputs; slope 24.57mV/dB, intercept -95.37dBm
When I made measurements with inputs set to -10dBm and -20dBm in the frequency range 10MHz-450MHz the output signal reduced by up to 10dB; please see attached graph.
This was completely unexpected and I was very surprised, as the data sheet contains the following statements viz, '-3dB bandwidth 900MHz', 'logarithmic linearity typically within +/- 0.4dB up to 100MHz' and '+/- 0.4dB linearity DC-400MHz'
So what is going on ? Why is the response not flatter ?